1. Field of the Invention
The present invention relates to exposure control devices for use in still cameras and the like, and more particularly to shutter control devices for automatically establishing exposure time in accordance with the level of scene illumination.
2. Description of the Prior Art
In many prior art shutter control devices which include timing circuits for establishing shutter speed, the circuit is energized by a battery contained in the camera. Such mechanisms usually include an electromagnetic transducer adapted to close the shutter after a period of time determined by the timing circuit. Although such arrangements are generally satisfactory, they rely on the availability of sufficient power from the battery. When the battery power is inadequate for operation of such systems, either the shutter operates at a fixed, predetermined speed or it remains open indefinitely, depending on the type of shutter control.
In our commonly assigned, co-pending U.S. Patent application Ser. No. 816,776, entitled PIEZOELECTRIC CAMERA SHUTTER, filed on July 18, 1977, a shutter control device was disclosed which employed a piezoelectric bimorph for providing electrical energy for powering an electronic exposure control circuit.
The shutter control device disclosed in that application represented a significant improvement over previously known mechanisms because piezoelectric devices are more reliable than battery sources and more efficient than conventional, non-battery powered sources known in the prior art for powering camera exposure control systems.
In that application, as well as in our commonly assigned, co-pending U.S. Patent application Ser. No. 816,834, entitled PIEZOELECTRIC ELECTRONIC SHUTTER CONTROL FOR CAMERAS filed on July 18, 1977, mechanical actuation of the shutter to terminate an exposure is effected by means of a piezoelectric motor which gradually changes shape as the electric field applied to the piezoelectric motor changes in accordance with the amount of light received by the film during the exposure. When a predetermined shape is attained by the piezoelectric motor, the shutter closes to terminate exposure. Since the shutter closing mechanism is dependent upon the position and shape of the piezoelectric motor, the location of the motor relative to the other related camera elements is critical to obtaining an accurate exposure time. Accordingly, extreme care must be taken during manufacture to insure that the piezoelectric motor is accurately positioned in the camera. Further in the latter two above-mentioned applications, there is an extent of rubbing between relatively moving parts during the shutter closing sequence. The amount of friction must be compensated for in positioning the piezoelectric motor during manufacture. Should wear during the camera's life change the amount of friction, the shutter timing might be affected.
By the present invention, a piezoelectric motor is arranged to supply mechanical energy to unlatch a camera's shutter in a manner in which the exact position of the motor is not as critical as in the prior art, and which thereby permits greater manufacturing tolerances during assembly and a reduced sensitivity to friction and wear during the camera's life.